Patent Document 1 discloses a technique having a valve opening/closing timing control apparatus (referred to as a “variable valve timing apparatus” in the above document) for setting opening/closing timings of an intake valve and an exhaust valve simultaneously. With this, the technique provides a control according to which when an engine load is below a predetermined value, closing timings of both the intake valve and the exhaust valve are retarded with maintaining an overlap between the intake valve and the exhaust valve constant.
Patent Document 2 discloses a technique having a valve opening/closing timing control apparatus (referred to as an “intake timing varying mechanism” in the above document) for setting an opening/closing timing of an intake valve and a valve opening/closing timing control apparatus (referred to as an “exhaust timing varying mechanism” in the above document) for setting an opening/closing timing of an exhaust valve, and a center timing of an overlap between the intake valve and the exhaust valve is varied according to a load.
Further, Patent Document 3 discloses a technique according to which a lift amount of an intake valve is varied by a variable lift mechanism. This Patent Document 3 describes that when the valve lift amount is varied, opening/closing timings of the intake valve are changed also.